The recent surge in printing beyond 2-D substrates into 3-D space to create so-called ‘smart objects,’ such as sensors and wearables, is leading to an increased market demand for electrically conductive materials that may be used to print circuit traces and other conductive elements on a range of substrates including plastics. Ideally, the electrically conductive material should also be capable of forming interconnects between electronic components and, e.g., the circuit traces.
Interconnects are the interfaces between conductors, e.g., wires or copper foil traces and electronic components, such as semiconductor devices, e.g., diodes, transistors and integrated circuits. Conventional, robust interconnects are typically formed using anisotropic conductive paste (ACPs), wire bonding or solder. ACPs include nonsolvent, liquid connection materials composed of structurally fine conductive particles diffused in a liquid thermosetting resin. These pastes may be used during Liquid Crystal Display (LCD) manufacturing and for surface mounting electronic components onto printed circuit boards (SMB).
Aluminum or gold wire bonding is also used to form interconnects. An example of how wire bonding is used is shown in FIG. 1. This figure provides a transistor 10, which comprises a silicon die 11, disposed on bonding pads (not shown) on a copper substrate 16. The silicon die 11 is connected by an aluminum bonding wire 13 via a wedge bond 12 to a source terminal 14 or a gate terminal 15. The wedge bond 12 is typically formed by using ultrasound to create a bond between the wire 13 and a terminal 14, 15.
Most typically, however, interconnects are formed using solders. Solders are generally prepared from fusible metal alloys having a melting point below the melting point of the metal parts that are to be joined. Solder is characterized by a melting behavior that does not change substantially with repeated heat/cool cycles. Adhesives and flux are often added to a solder to form a paste, which is disposed at the point of contact to hold the metals in place until the solder is melted or “reflowed” in an oven to make the final connection. The flux in the solder paste is used to promote fusing of the metals and removes and prevents the formation of nonconductive metal oxides, which may reduce the reliability of a soldering connection.
Electronic components may be electrically connected to, e.g., a circuit board using solder balls or bumps. As shown in FIG. 2, for example, solder bumps 28 are used to mount an integrated circuit 25 onto a printed circuit board 29. More particularly, FIG. 2 depicts a flip chip 20, which includes a semiconductor die or integrated circuit 25, which is mounted onto a circuit board 29 via solder bumps 28 coated with flux 26 with the active side of the integrated circuit 25 facing the circuit board 29. The flip chip interconnection is made by contacting the solder bumps 28 of the integrated circuit 25 with corresponding interconnect sites 30 on the circuit board circuitry and then heating to reflow the fusible portion of the solder bumps 28 to make the electrical connection. An underfill 27, such as epoxy, may be used to fill the area between the integrated circuit and the circuit board for additional mechanical strength.
Although solder bumps, ACPs and wire bonds may be effectively used to form interconnects, the processes using these materials generally require high temperature and pressure. Accordingly, interconnects are typically formed on rigid substrates, such as silicon. Consequently, the use of conventional interconnect materials is likely to be of limited use for flexible printed electronics, which typically require flexible plastic substrates that melt at low temperatures, e.g., 150° C.
Other electrically conductive materials known in the art that may be used at low melting temperatures and, thus, may be suitable for use on a wide range of substrates including flexible plastic substrates, may not be suitable for forming interconnects since they often poorly adhere to electronic components. For example, as shown in FIG. 3, nanosilver inks, which typically have a melting temperature (≤145° C.) much lower than the bulk metal, are capable of forming conductive elements by bonding (sintering) the silver particles at low temperature. Owing to their low viscosity and high silver content, these ink materials may be deposited on a substrate using a jetting application. Further, nanosilver inks are capable of forming thick films of up to 10-20 μm. Despite these benefits, however, nanosilver inks often do not adhere well to electronic components, thus limiting their use as interconnects. Moreover, nanosilver inks are expensive, further limiting their use.
Liquid metals have also been identified as potentially useful materials for flexible printed electronics. See Joshipura et. al. J. Mater. Chem C. (2015), 3, 3834-3841. These metals, which include Gallium, Indium, Bismuth, and Tin, typically have melting points ranging from 10-150° C. Due to their low-melting behaviors, research concerning the use of liquid metals has involved their encapsulation in an effort to control the flow of these materials. See Gozen et. al. Adv. Mater. (2014), 26, 5211-5216. For example, FIG. 4 depicts the fabrication of a conductive pattern using a liquid metal and encapsulation of the liquid metal within microchannels. As shown in this scheme, microchannels are molded onto the surface of an elastomer, e.g., poly(dimethylsiloxane) (PDMS) and filled with liquid metal, e.g., eutectic gallium indium (EGaIn). After sealing with an additional layer of PDMS, the liquid-filled channels can function as stretchable circuit wires. Nevertheless, these metals may not be ideal for use as interconnects since a consistent way for electrically contacting the liquid metals to electronic components has not been identified. See Joshipura et. al. J. Mater. Chem C. (2015), 3, 3834-3841, 3839-3840.
Consequently, in view of the above, there remains a need in the art for materials that demonstrate good electrical conductivity, are less expensive than nanosilver inks and which are suitable for fabricating interconnects as well as conductive features such as traces, electrodes and the like on a variety of substrates, including plastics.